1. Field of the Invention
The present invention relates to a blood filter used for forming an extracorporeal circulation circuit for extracorporeally circulating bodily fluids such as blood in, for example, an artificial cardiopulmonary system and an artificial kidney system. More particularly, the present invention relates to a blood filter for removing foreign substances, thrombus and air bubbles in an extracorporeal circulation circuit.
2. Description of Related Art
Extracorporeal circulation of bodily fluid such as blood is generally applied to patients who require heart surgery or artificial dialysis. In extracorporeal circulation in heart surgery, the patient""s blood is guided outside the body and treated by an artificial cardiopulmonary system provided outside the patient""s body, and the treated blood is returned to the body. Extracorporeal circulation is generally performed using an extracorporeal circulation circuit including a chamber (or blood reservoir), a blood pump, an artificial lung, a filter and a tube.
One serious problem in extracorporeal circulation of blood is formation of a thrombus. A thrombus is formed by contact of the blood with a foreign substance, such as air bubbles mixed in the extracorporeal circulation circuit or a tube included in the extracorporeal circulation circuit. Specifically, the air mixed in the extracorporeal circulation circuit activates a blood coagulation system and promotes formation of the thrombus. Accordingly, an extracorporeal circulation circuit generally includes a device for removing air bubbles mixed therein. Usually, the air bubbles mixed in the extracorporeal circuit are primarily removed in a blood reservoir. The thrombus is also formed by contact of blood and a foreign substance, such as a plastic material used for the extracorporeal circulation circuit.
In order to remove the air bubbles mixed in the extracorporeal circulation circuit, an open-type blood reservoir is in a wide use, which is easy to operate and has a high air bubble removing ability. The open-type blood reservoir includes a region of air therein so as to provide a large contact area between the blood and the air. Due to such a structure, the air bubbles mixed in the extracorporeal circulation circuit is moved to the region of air with certainty and thus the air is removed from the extracorporeal circulation circuit A closed-typed reservoir having no region of air therein has also been developed and marketed. It is difficult to remove air bubbles completely from the extracorporeal circulation circuit which circulates a large amount of blood and includes either type of blood reservoir. Thus, the air bubbles which have not been removed in the blood reservoir are trapped by a blood filter and removed before returning to the patient""s body.
Foreign substances derived, for example, from the tube, blood pump or artificial lung included in the extracorporeal circulation circuit can also be mixed in the extracorporeal circulation circuit.
The thrombus and the foreign substances contained in the extracorporeal circulation circuit are generally removed by a blood filter in a final stage of the extracorporeal circulation circuit before the blood is returned to the patient""s body.
The blood filter for removing air bubbles and thrombus contained in the extracorporeal circulation circuit is required to have a minimum possible volume while maintaining a low pressure loss with respect to the blood flow rate required for the extracorporeal circulation circuit and in order to minimize the priming volume in the extracorporeal circulation circuit. When the priming volume of the blood filter is reduced, however, the ability of removing the air bubbles, the thrombus and the other foreign substances is lowered and the pressure loss is increased.
Especially in recent cardiovascular surgery, it has been actively attempted to minimize surgical invasion to the patient so as to promote recovery and minimize side effects on the patient. One type of maneuver referred to as minimally invasive surgery has been developed. It has also been actively attempted to minimize the priming volume in the extracorporeal circulation circuit so as to farther suppress the side effects caused by the dilution of the blood and blood transfusion. In accordance with such trends, the components incorporated in the extracorporeal circulation circuit are now required to be a further reduced in size and volume.
Conventionally, the extracorporeal circulation temperature of the blood during surgery is usually maintained low for the purpose of reducing oxygen consumption by the patient However, a low temperature leads to increased operation time due to the extended time needed to re-raise the temperature of the blood to normal body temperature and organopathy is caused by low temperature circulation. Thus, it has been actively attempted to maintain the temperature of extracorporeally circulating blood at a level which is as close as possible to the body temperature. This helps to minimize invasion to the patient, as well as shorten the operation time. In the case where the blood temperature is maintained low, the extracorporeally circulating blood needs to be set in a larger amount than is required by the consumption by the brain and main organs of the patient during the surgery. Accordingly, the components in the extracorporeal circulation circuit need to maintain a sufficiently high performance even with a high flow rate of blood.
Under the circumstances, a blood filter which is compact and still has a sufficiently high removing ability of air bubbles, thrombus and foreign substances and a low pressure loss even with a high flow rate of blood has been demanded.
The present invention has an objective of improving the conventional blood filter used for an extracorporeal circulation circuit and providing a blood filter which eliminates the problems unsolved by the conventional technology and handles the above-described new method of surgery. In other words, the present invention has an objective of providing a blood filter which is a compact and still has a sufficiently high removing ability of air bubbles, thrombus and foreign substances and a sufficiently low pressure loss even with a high flow rate of blood.
In order to achieve the above-described objective, the blood filter according to the present invention includes a housing having a spiral chamber with a blood inlet at one end thereof, preferably in the vicinity of a bottom surface of the housing. The spiral chamber is defined between a partially cylindrical spiral-shaped inner wall and a cylindrical outer wall of the blood filter housing and extends along the inner wall of the housing.
More specifically, the blood filter of the present invention comprises: a housing accommodating a blood inlet, a blood outlet, an air bubble outlet; an inner wall, an outer wall, and a filter element disposed in the housing. The filter element divides an inner space of the housing into a first space which is in communication with the blood inlet and a second space which is in communication with the blood outlet. The first space includes a center chamber defined within the inner wall of the housing and a spiral chamber defined between the inner wall and the outer wall of the housing and extending along the inner wall of the housing. The spiral chamber has the blood inlet at one of two ends hereof and an opening at the other end thereof which is in communication with the center chamber. The center chamber has the air bubble outlet. The spiral chamber has an inner bottom surface and an inner top surface which define a spiral passage extending upwardly from the blood inlet to the opening. The spiral passage is disposed to substantially surround the center chamber.
The inner bottom surface of the spiral chamber may extend along a perimeter of a bottom surface of the housing. An inner surface of the filter element which faces the second space and a part of an inner surface of the housing defines a lengthy passage which has the blood outlet at one end thereof and extends perpendicular to the inner bottom surface of the spiral chamber.
Preferably, the spiral passage of the spiral chamber surrounds the center chamber in the range of 180 degrees to 400 degrees.
Preferably, an apex of the inner top surface of the spiral chamber, the apex being in the vicinity of the opening, is at a higher level than the filter element.
Preferably, the spiral passage of the spiral chamber has a cross-section which increases from the blood inlet to the opening.
Preferably, the spiral passage of the spiral chamber has a cross-section which increases so that a blood velocity at the opening is xc2xd or less of a blood velocity at the blood inlet.
Preferably, the inner top surface of the spiral chamber extends from the blood inlet to the opening in a direction away from the inner bottom surface of the spiral chamber.
Preferably, the inner top surface of the spiral chamber extends at an angle in the range of 5 degrees to 60 degrees with respect to the inner bottom surface of the spiral chamber.
Preferably, the distance between the apex of the inner top surface of the spiral chamber in the vicinity of the opening and the inner bottom surface of the spiral chamber may be about 4 times or more the width of the spiral chamber in a radial direction of the housing.
The center chamber may have a sub chamber and the air bubble outlet may be disposed at an apex portion of the sub chamber.
Preferably, the sub chamber has a volume which is {fraction (1/100)} to {fraction (1/10)} of the volume of the center chamber.
Preferably, the filter element is disposed in the center chamber in such a manner that the center axis of the filter element does not correspond to the center axis of the center chamber.